Electrical wires for use in industrial machines such as computers, office-use equipments, and vehicles and in home-use electronic machines such as audio equipments, video recorders and players, and personal computers and electrical wires for house wiring and other uses are recently required to have good flame-retardant properties along with the property of not evolving harmful gases during combustion. Along these requirements, insulating tubings for the protection or terminal treatment of those electrical wires are also coming to be required to have good flame retardant properties along with the property of not evolving harmful gases during combustion.
A known expedient for attaining the required flame retardancy is to incorporate a large amount of magnesium hydroxide, which is a flame retardant not containing a halogen, into a thermoplastic resin such as a polyolefin (JP-B-62-181, JP-B-57-10898). (The term "JP-B" as used herein means an "examined Japanese patent publication".)
However, flame-retarded resin compositions obtained by incorporating a large amount of magnesium hydroxide into thermoplastic resins such as polyolefins have been unsuited for use in applications such as insulated electrical wires and insulating tubings, because they have a problem of low volume resistivity and also have a problem in that their initial tensile strengths are low and their properties are impaired significantly through heat aging.
For example, the volume resistivity of polyolefin-based heat-shrinkable tubings should be 10.sup.- .OMEGA.cm or more according to the UL (Underwriters Laboratories) Standards. In the case of polyolefin resin-insulated electrical wires, the UL Standards prescribe that the initial tensile strength be 1.06 kg/mm.sup.2 or more and the residual elongation at break after heat aging (percent retention of elongation at break after heat aging) be 65% or more. However, such a flame-retardant material containing no halogen is not known that satisfies heat aging characteristics requirements such as the UL rating of 105.degree. C. (residual initial tensile strength at break after 7-day heat aging at 136.degree. C. of 70% or more and residual elongation at break after the same heat aging of 65% or more) and the UL rating of 125.degree. C. (residual initial tensile strength at break after 7-day heat aging at 158.degree. C. of 70% or more and residual elongation at break after the same heat aging of 65% or more) as well as the above-described volume resistivity and initial tensile strength requirements.
Of the above-described problems, only the volume resistivity problem has been eliminated by use of a resin composition obtained by incorporating from 100 to 250 parts by weight of magnesium hydroxide and from 5 to 50 parts by weight of clay into 100 parts by weight of a thermoplastic resin such as an ethylene-vinylacetate copolymer or an ethylene-ethylacrylate copolymer thereby to improve volume resistivity without impairing the flame retardancy of the resin, as disclosed in JP-A-63-260957. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)